Major industrial methods to produce silicone rubbers are:
the peroxide cross-linking of vinyl containing silicone prepolymers,
the tin salt catalyst induced cross-linking of silanol containing silicone prepolymers and
the platinum salt induced crosslinking of vinyl and SiH containing silicone prepolymers.
The first two methods do not give medical grade silicone rubbers as small amounts of cross-linking chemicals and reaction products are still present in the rubber. These residues are no problem for non medical applications. The third method, the platinum salt catalyzed cross-linking, is the preferred method to obtain medical grade silicone rubbers.
The molecular design and synthesis of hydrophilic silicone materials is a relatively unexplored area. Still, some hydrophilic silicone materials have been disclosed in the known prior art. For example, patent application US2002/0160139 discloses a surface modified polymer including a surface that is covalently bonded to a surface modifying compound. Formation of the covalent bond between the polymer and the surface modifying compound is achieved by a reaction between an intrinsic functional group that is present in the polymer and the functional group of the surface modifying compound. By using a polymer having an intrinsic functional group, a separate surface activation step is avoided. Thus, the material has a hydrophilic surface while the bulk of the material remains hydrophobic. Accordingly, this material does not allow for the uptake of moisture through the material and moisture can thus not be removed effectively.
WO 2010/095105 discloses, for use in a microfluidic system, a rubber material comprising polar side groups whereby each of the side groups is linked with the polymer chain of said rubber material via a linker comprising at least 6 atoms. The polar side groups may be ionic side groups such as —SO3-. For instance the material may be a silicone rubber modified with 15-20 w % sodium alkene C14-16 sulfonate. The silicone rubber may have a chain length from 1000 to 10000 Si—O units, and the modified silicone rubber may be made by radical addition of ω-alkenylsulfonic acids to siloxane units present in the polysiloxane chain.
More generally, many commercial polymers are not skin friendly as they do not absorb water or sweat from the skin.
Introduction of an alpha-olefin sulfonate surfactant into these polymers may provide a copolymer with an increased hydrophilic character which can be used to increase its biocompatibility and its capacity to hold water. For the manufacture of skin-contact products, this is especially relevant to biocompatible polymers such as, but not limited to, silicones, polybutadiene, polybutadiene-containing polymers, polybutadiene-polyethylene oxides copolymers, poly(meth)acrylates, and isobutylene-ethylene glycol copolymers.
However alpha-olefin sulfonate surfactants, although having a vinyl functional group, do not easily mix with the monomer of commercial polymers like polyethylene (PE), polypropylene (PP), polybutadiene, polyisoprene, polystyrene (PS), polyacetonitrile (PAN), silicones, poly(meth)acrylates, polyacrylonitrile, acrylonitrile-butadiene-styrene copolymers (ABS) and styrene-acrylonitrile copolymers (SAN). This incompatibility can be due to differences in boiling points, making these non volatile surfactants nearly impossible to use in gas phase polymerizations. Even under liquid phase polymerization conditions, it is difficult to mix a hydrophilic surfactant containing a sulfonic acid salt with a hydrophobic monomer or pre-polymer. Only in a special case like the suspension polymerization of vinyl chloride in water, can the hydrophilic surfactant be dissolved in a part of the reaction mixture (water) and thus incorporated into the main polymer. However polyvinyl chloride is not regarded as a skin-compatible polymer.
In front of the above mentioned prior art, there is still a need for a material with hydrophilic properties for use as an aid in moisture control, in particular for manufacturing products with moisture control properties such as, but not limited to, skin-contact products or mucosa-contact products.
In order to manufacture rubber materials being modified with an alpha-olefin sulfonate surfactant, there is also a need in the art for facilitating the miscibility of said alpha-olefin sulfonate surfactant with hydrophobic organic monomers or pre-polymers under conventional liquid phase polymerization conditions.
There is also a need in the art for conveniently producing rubbery or elastomeric polymer materials with hydrophilic properties for use in non-medical applications wherein high water-uptake capacity is desirable. For instance, with respect to silicone rubbers, the production of silicone rubbers with hydrophilic properties should not be limited to the platinum salt catalyzed cross-linking method specified hereinabove.